An exemplary birefringence measurement system is described in U.S. Pat. No. 6,985,227. A schematic diagram of the optical setup 20 of one embodiment of such a system is shown in FIG. 1. That system employs as a light source 22 a polarized He—Ne laser having a wavelength of 632.8 nm. The light beam B1 from the source 22 passes through a polarizer 24 oriented at 45 degrees. The system also includes a photoelastic modulator (“PEM”) 26 oriented at 0 degrees and operated at 50 KHz.
A sample holder 34, which can be mounted on a computer-controlled X-Y stage is provided, thereby to allow the beam to scan various locations of an optical element or sample 36.
The light beam 82 that emanates from the sample (for convenience hereafter referred to as the “sampled beam” B2) is directed through another PEM 28 that is oriented at 45 degrees and operated at 60 KHz. After passing through an analyzer 30 oriented at 0 degrees, the sampled beam B2 is directed into the receiving or active area of a Si-photodiode detector 32.
The light source beam (in this case, laser light) is well collimated, and compact detectors (that is, having relatively small active areas on the order of 1-6 mm) may be employed, especially where the sample 36 has parallel surfaces normal to the incident light beam B1. Also, birefringence characteristics of thin samples, such as thin films, may also be effectively measured with such a system, even though the sample may be tilted relative to the source beam, such that the incident angle of the beam is oblique to the surface of the sample.
There are certain applications, however, where the sample thickness or shape will have the effect of refracting the beam B1 in a manner such that the sampled beam B2 emanating from the sample will not align with the active area of the detector. In this regard, reference is made to FIGS. 2A-2C.
FIGS. 2A-2C show as a module 40 the combined light source 22, polarizer 24 and PEM 26 mentioned above. Similarly, the second PEM 28, analyzer 30 and detector 32 are shown as a single module 42. FIG. 2A shows a collimated light beam B1 emanating from a generally parallel-surfaced sample 36A that is oriented with its surfaces normal to the incident light beam B1. The sample will not significantly alter the direction of the incident beam, and the sampled beam B2 will thus be incident upon the active area 44 of the detector, which is essentially aligned with the light source beam B1.
FIG. 2B illustrates an application where, for example, a relatively thick sample 36B is held so that it is tilted as respects the incident beam B1, which has the effect of redirecting the sampled beam B1 so that beam does not impinge upon the active area 44 of the detector. A similar misalignment is shown in FIG. 2C where a wedge-shaped sample 36C has the effect of realigning the sampled beam B2 by an amount such that the active area 44 of the detector fails to detect the intensity or other characteristics of the beam B2. The signal processing components of the birefringence measurement system noted above require this information for analyzing the birefringence characteristics of the sample.
The present invention provides a simple, versatile, and low-cost technique for ensuring that the information carried in the sampled beam B2 is directed to the active area of the detector, thereby enabling effective birefringence measurement of a variety of sample shapes and sizes.